Field of the Invention
This invention relates to electrode configurations for gas discharge devices and more particularly to an electrode configuration adapted for providing a laser gain medium having dual discharges for use in a differential laser gyro.
Ring type lasers employing electromagnetic waves at optical or near optical frequencies traveling in both a clockwise and a counter-clockwise direction about a closed path in a principal plane, have been utilized to sense rates of angular rotation similar in function to the well known electromechanical gyro. The rotation of the gyro requires more time for a traveling wave to complete the optical path in the direction of rotation and requires less time for a traveling wave to complete the optical path in a direction opposite to the direction of rotation. Thus, if the gyro is rotating in the same direction as the clockwise beam, the frequency of the clockwise beam will appear to be less than a natural frequency of the laser, whereas the frequency of the counter-clockwise will seem to be higher. The difference between the two frequencies is a function of the rate of rotation of the gyro.
Laser gyros typically impose severe restraints on the gain tube of the laser. The gain tube must typically have small dimensions for incorporation within one leg of the ring laser. Additionally, the gain tube is typically enclosed within a gyro block having a low distortion coefficient, such as the coefficient of expansion, and having poor heat transfer characteristics. The heat generated within the gain tube during operation must be efficiently extracted through the gyro block to minimize frequency distortions in the clockwise and counter-clockwise waves. Since the gyro block has poor heat transfer characteristics, the gain tube must be designed to produce a miniumum amount of heat during operation to avoid distortion of the tube and plasma instabilities within the gain medium resulting in frequency distortions.
Additionally, the clockwise and counter-clockwise waves tend to mode lock, that is, the waves interfere with one another to produce waves having identical frequencies. In addition, one of the traveling waves, by virtue of intrinsic or momentary power loss within the gain medium, may achieve dominance over the other waves and deplete the gain population sufficiently to extinguish the other traveling waves. This effectively results in a deadband for low angular rates of the gyro.
To reduce the probability of interactions occurring between the modes, it is necessary to operate the gain tube with a lasing gas having a low pressure. This is particularly true when the lasing gas is a mixture of helium and neon. Additionally, the low pressure also reduces broadening of the lasing lines which contributes to improved performance and minimizes laser frequency shift resulting from gas pressure variations within the gain area. However, in prior art devices low pressure operation of gain tubes typically resulted in cathode sputtering which seriously degraded the gain tube lifetime. Cathode sputtering is further enhanced by the high current densities incident onto the cathodes resulting from the small effective surface area of the cathode which is required to obtain a gain tube having small dimensions.
One method well known in the art of increasing the effective surface area of a cathode and thereby reducing the current density is the utilization of a hollow cylindrical cathode with a discharge tube extending partly into the cylindrical cathode symmetrically about a longitudinal axis of the cylinder. In operation, the discharge flows from an anode exterior to the discharge tube, through an inner passage within the tube to the interior cylindrical wall of the cathode. The discharge tube extends partly into the cylindrical cathode to reduce sputtering which may occur at the ends of the cylinder. The interior diameter of the cylinder is sized to have an effective surface area sufficient to produce a low surface current density incident thereon. This configuration results in some reduction in the current density incident onto the cathode surface but a high current density persists on that portion of the cathode surface in close proximity to the end of the discharge tube resulting in a relatively high rate of cathode sputtering with a corresponding gain tube deterioration.